This invention constitutes an improvement over U.S. Pat. No. 5,486,093 granted to Auxier et al on Jan. 23, 1996 entitled LEADING EDGE COOLING OF TURBINE AIRFOILS. This patent teaches the use of helix shaped cooing passages in the leading edge of the turbine blade so as to enhance convective efficiency of the cooling air and to improve discharge of the film cooling air by orienting the discharge angle so that the discharging air is delivered more closely to the pressure and suction surfaces. The helix holes place the coolant closer to the outer surface of the blade to more effectively reduce the average conductive length of the passage so as to improve the convective efficiency. Also higher heat transfer coefficients are produced on the outer diameter of helix holes improving the capacity of the heat sink. This patent is incorporated herein by reference.
U.S. Pat. No. 4,180,373 granted to Moore et al on Dec. 25, 1979 and entitled TURBINE BLADE, U.S. Pat. No. 5,356,265 granted to Kercher on Oct. 18, 1994 entitled CHORDED BIFURCATED TURBINE BLADE, U.S. Pat. No. 5,967,752 granted to Lee et al on Oct. 19, 1999, and U.S. Pat. No. 5,538,394 granted to Inomata et al on Jul. 23, 1996 exemplify traditional techniques for cooling the airfoil leading edge. In the teachings of these patents, the airfoil leading edge is cooled with backside impingement in conjunction with showerhead film cooling. Showerhead film cooling holes formed in rows spanning the leading edge along the radial and chord-wise axis are fed coolant from a common mid-chord cavity so as to direct impingement air on the back wall of the leading edge and feed the film cooling holes. The coolant discharges from the blade at various pressures of the engine working medium that is adjacent the discharge of the film cooling hole. As a result of this cooling approach, cooling flow distribution and pressure ratio across the showerhead film holes for the pressure side and suction side is predetermined by mid-chord cavity pressure. This condition is more clearly shown in FIG. 4 which is a graph plotting the airflow of the air extending a distance spanning the suction side to the pressure side. Since the pressure of the engine working fluid closer to the suction side of the blade is less than the pressure adjacent to the pressure side as the coolant flows through the rows of blade spanning the leading edge from the suction side to the pressure side, there is a drop off of airflow as represented by the solid line in FIG. 4.
In addition, the conventional film cooling holes pass straight through the airfoil wall at a constant diameter and exit at an angle to the exterior surface. Some of the coolant is subsequently injected directly into the mainstream causing turbulence, coolant dilution and loss of downstream film cooling effectiveness. Furthermore, film cooling hole breakout on the airfoil surface may induce stress problems. For further details of the operation of shower head cooling for turbine blades reference should be made to U.S. Pat. Nos. 4,180,373, 5,356,265, 5,967,752 and 5,538,394, supra, all of which are incorporated herein by reference.
This invention not only serves to alleviate the problems noted in the above paragraph, but provides cooling with a lesser amount of cooling air which improves the efficiency of the turbine an adds to the performanc of the engine. In accordance with this invention, the leading edge is cooled by film cooling by first diffusing the coolant before being discharged out of the blade. The diffusion is accomplished by controlling the pressure ratio across the film cooling hole by first passing the coolant through a first restriction and then a second restriction to obtain the desired pressure and then discharging the coolant into an elongated chamber formed on the outer surface of the leading edge. The restrictions are located upstream of a plenum chamber where the coolant is diffused and ultimately into an elongated chamber or pocket formed on the exterior wall of the leading edge. These chambers are arranged in an array of parallel spaced columns and rows thereof extend along the leading edge and may be aligned in the chord-wise direction or stepped radially. These pockets have a twofold purpose, namely 1) they provide an insulation blanket of cooled air to cool the surface of the leading edge and 2) they remove the metal surface of the leading edge and hence the path of heat conductivity is lessened.